That should work for you. If your using VS 2005
VB Code:
  1. Public Sub Command1_Click(ByVal Sender as System.Object,ByVal e as System.EventArgs) Handles Button1.Click
  2. if ComboBox1.Items.Contains(ComboBox1.Text) = False then
  3.  ComboBox1.Items.Add(ComboBox1.Text)
  4. End If
  5. End Sub